visible, and a piece of paper thrown into the aperture did not rise again, so that there was every reason to suppose the existence of a descending current of air. The subterraneous thunder heard at such great distances under Vesuvius, is almost a demonstration of the existence of great cavities below, filled with äeriform matter; and the same excavations which in the active state of the volcano throw out immense volumes of steam during so great a length of time, must in all probability, become filled with atmospheric air in its quiet state.

The limestone caverns of Carniola, some of which contain many hundred thousand cubic feet of air, show the vast extent to which subterraneous cavities may exist, even in common rocks; and the deeper the excavation, the denser is the air, and the fitter for combustion.

The same circumstance which would enable the alloys of the metals of the earths to produce volcanic phenomena, namely their extreme facility of oxidation, must likewise prevent them from being ever found in a pure combustible state among the products of volcanic eruptions; for before they reach the external surface, they must not only be exposed to the air in the subterranean cavities, but be propelled by steam, a substance equally fit to oxidize them as the air. 66 Assuming the hypothesis of the existence of such alloys of the metals of the earths as may burn into lava in the interior, the whole phenomena may be easily explained from the action of the air and the water of the sea, on those metals; nor is there any fact, or any of the circumstances which I have mentioned in the pre

TRUE SOURCE OF VOLCANOES TRACED. 401

ceding part of this paper, which cannot be easily explained according to that hypothesis. For almost all the volcanoes in the old continent of considerable magnitude are near to, or at no considerable distance from, the sea; and, if it be assumed that the first eruptions are produced by the action of sea water upon the metals of the earths, and that considerable cavities are left by the oxidated metals thrown out as lava, the results of their action are such as might be anticipated; for after the first eruptions, the oxidations which produce the subsequent ones may take place in the caverns below the surface. When the sea is distant, as in the volcanoes of South America, they may be supplied with water from great subterraneous lakes; as Humboldt states that some of them throw up fish.

"On the hypothesis of a chemical cause for volcanic fires, and reasoning from known facts, there appears to me no other adequate source than the oxidation of the metals which form the bases of the earths and alkalis."-Sir H. Davy, ut supra.

We must also take into account, considerations derived from thermometrical experiments on the temperature of mines and hot water springs, which show that the interior of the globe possesses a very high temperature. Thus its nucleus would appear to be in fluid ignition; whence the solution of the problem of volcanic fires, becomes still more simple and satisfactory.-See beginning of Chap. iii.

To this head belongs the great family of trap rocks, called by geologists, unstratified superjacent, which cover and conceal the secondary beds over immense tracts of country, and which may be regarded as relics of those tremendous volcanic operations, which often shook the old world, while they ushered in, caused, and accompanied the deluge. Nowhere do these eruptive monuments display more instructive and magnificent forms, than in the North of Ireland, and among the Scottish Hebrides, though in one shape or another, they are scattered over all the earth, attesting the universality of submarine fires in the primeval globe. From the manifest tokens of protrusive violence which this formation has every where left among the secondary and tertiary beds of limestone, sandstone, chalk, &c. between and above which it is seen diffused, there can be no doubt that it was projected up through

CHARACTER OF A BASALTIC REGION.

408

them, and thrown over them, long after they were deposited, and is therefore clearly of posterior date. For the valuable details which follow, I am principally indebted to Dr. Macculloch and Dr. Boue.

The overlying rocks appear under various aspects, to which mineralogists have given a variety of names; but in truth they all graduate by insensible shades into each other, and hence seem to have had a common origin, as they are in chemica composition nearly the same. They present themselves often in abrupt columnar masses of gigantic dimensions, imparting a black and dismal character to the territory over which they preside. These columns sometimes stand vertically over tremendous precipices, at others, they stretch horizontally along the cliffs, and now and then are twisted and bent into the most curious curvatures. Their blocks are often piled up in rectangular masses, retiring behind each other on the face of a mountain range, as if composing the steps of some colossal stair; whence the generic term trap (stair) was applied to them in Sweden, a name now universally adopted.

This family of rocks, though they want the continuous expanse of the stratified class, and therefore can hardly be ranked among general formations, are yet very extensively distributed over the face of the earth. If Great Britain, for example, in its middle and southern districts, be nearly destitute of them, near the surface, its coal-measures throughout exhibit abundant marks of their presence; and a very large proportion of Scotland lies under their influence.

Here trap sometimes rises into considerable mountains, as in Mull, or forms stupendous mural precipices, as along the western shores of Sky. Occasionally it spreads over the secondary beds in a vast expanded sheet, or shoots out between their layers in a pseudo-stratification; phenomena well exemplified on the whole Antrim coast, from Larne to the Giant's-Causeway. The basaltic effusions are superposited on formations belonging to every geological age, from granite and gneiss to tertiary strata. In Auvergne they are observed reposing on the latest mineral beds, the freshwater limestone; near Annaberg in Saxony, they lie over diluvial land; and in the Vivarais, at the summit of the Coirons, M. Beudant discovered them resting on strata, which contain land-shells (cyclostoma elegans), whose analogues still live in the same country. Nowhere have they been found covered with other mineral beds; and therefore we are justified in regarding the greater part of them as of posterior origin. On the other hand, the appearance of the fragments of these rocks in the lowest sandstone conglomerate, proves that some trap formations also existed before the deposition of the secondary strata.

Granite is sometimes penetrated with veins of trap as at the Cape of Good Hope, where numerous veins of basalt pervade it, which vary in diameter from an inch to ten feet, and branch off in as many directions, as granite veins do in gneiss, but more frequently it is surmounted by coulées of it. Among stratified rocks, both medial and supermedial, it is found in broad masses, as well as in